The application relates to a bone anchoring assembly for dynamic stabilization of bone or vertebrae.
A bone anchoring assembly with a flexible rod made of an elastomer material is known, for example, from EP 1 759 646 A1. The rod is held in the receiving part by means of a closure cap and a filling piece which presses onto the rod when the closure cap is screwed onto the receiving part. The surface of the filling piece and the bottom of the receiving part includes, for example, conical pins which press onto the rod and create an indirect form-fit connection which contributes to the frictional connection so as to hold the rod safely in place. The indirect form-fit connection is achieved by a local elastic or plastic deformation of the material of the rod. The bone anchoring element is of the type of a monoaxial screw, wherein the receiving part and the shank are not pivotably connected.
A bone anchoring assembly with a flexible rod is further known from EP 1 795 134 A1 which describes a polyaxial bone anchoring element. The receiving part and the shank are pivotably connected and a pressure element is provided to lock the angular position of the shank relative to the receiving part. The surface of the filling piece and that of the pressure element which contacts the rod has rib-like projections which press onto the flexible rod and provide a form-fit contribution to the fixation of the rod in the receiving part.
From EP 1 900 334 A1 a bone anchoring assembly of the above mentioned type is known, which includes a single part closure element instead of a closure element with a filling piece. The single part closure element is an inner screw to be screwed between the legs of the receiving part, which has an annular projection on its lower side which presses onto the flexible rod.
The bone anchoring assemblies mentioned above which use the flexible rod comprise an engagement structure to clamp the rod which has sharp edges and/or which has teeth or ribs which are arranged exactly on opposite sides of the rod in order to provide a safe locking.
With such engagement structures there is a risk of weakening the rod, if the rod diameter is small. Therefore, the known assemblies are mainly used with rods having a relatively large diameter, for example a diameter of approx. 9 mm or larger. However, there is a need for the use of bone anchoring assemblies of the type using a flexible rod which are small in size, in particular, when the implant is to be placed at a location which is exposed and not covered enough by muscles, ligaments or other soft tissue.
FR 2 624 720 discloses an osteosynthesis device including a bone anchoring element with a shank and a receiving part and a fixation rod which is not flexible but rigid, usually made of a biocompatible metal. The fixation rod is held in the receiving part by a locking cap which includes a central pin with a tip penetrating into the surface of the rigid rod. The pin with the tip serves for a provisional fixation of the rod until the locking cap is tightened.
Based on the foregoing, there is a need for a bone anchoring assembly for dynamic stabilization using a flexible rod which is smaller in size compared to the known bone anchoring assemblies while providing the same degree of safe fixation of the rod as the known bone anchoring assemblies.